Multi-junction solid state transducer devices for direct ac power and associated systems and methods

ABSTRACT

Multi junction solid-state transducer (SST) devices and associated systems and methods are disclosed herein. In several embodiments, for example, an SST system can include a first multi-junction SST chain having a first drive voltage, a first P-contact, and a first N-contact, and a second multi junction SST chain having a second drive voltage, a second P-contact, and a second N-contact. The first and second multi junction SST chains can be configured to be activated independently of each other. The SST system can further include a driver operably coupled to the first and second P- and N-contacts. The driver can be configured to activate the first multi junction SST chain when voltage input is at least equal to the first drive voltage. When absolute voltage increases a predetermined voltage level, the driver can be configured to activate the second multi-junction SST chain or the first and second multi junction SST chains.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of U.S. application Ser. No. 13/923,647filed Jun. 21, 2013, now U.S. Pat. No. 9,326,338, which is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

The present technology is related to solid state transducer (“SST”)devices and methods of manufacturing SST devices. In particular, thepresent technology is related to multi junction SST devices for use withalternating current (“AC”) power and associated systems and methods.

BACKGROUND

Mobile phones, personal digital assistants (“PDAs”), digital cameras,MP3 players, and other electronic devices utilize light-emitting diodes(“LEDs”), organic light-emitting diodes (“OLEDs”), polymerlight-emitting diodes (“PLEDs”), and other solid-state transducerdevices for backlighting. Solid-state transducer devices are also usedfor signage, indoor lighting, outdoor lighting, and other types ofgeneral illumination. FIG. 1A is a cross-sectional view of aconventional LED die 10 a with lateral contacts. As shown in FIG. 1A,the LED die 10 a includes a substrate 20 carrying an LED structure 11having an active region 14, e.g., containing gallium nitride/indiumgallium nitride (GaN/InGaN) multiple quantum wells (“MQWs”), positionedbetween N-type GaN 15 and P-type GaN 16. The LED die 10 a also includesa first contact 17 on the P-type GaN 16 and a second contact 19 on theN-type GaN 15. The first contact 17 typically includes a transparent andconductive material (e.g., indium tin oxide (“ITO”)) to allow light toescape from the LED structure 11.

FIG. 1B is a cross-sectional view of another conventional LED die 10 bin which the first and second contacts 17 and 19 are opposite eachother, e.g., in a vertical rather than lateral configuration. Duringformation of the LED die 10 b, the N-type GaN 15, the active region 14and the P-type GaN 16 are stacked sequentially on a growth substrate(not shown), similar to the substrate 20 shown in FIG. 1A. The firstcontact 17 is formed on the P-type GaN 16, and a carrier substrate 21 isattached to the first contact 17. The growth substrate is then removedand the second contact 19 is formed on the N-type GaN 15. The structureis then inverted to produce the orientation shown in FIG. 1B.

Many LED devices (e.g., including the LED dies 10 a and 10 b of FIGS. 1Aand 1B) are operated using AC power rather than direct current (“DC”)power because it reduces the complexity of the power supply electronicsand, accordingly, can reduce the overall cost of the device (e.g., byremoving the AC/DC converter). Typical AC-driven LED devices include LEDdies (e.g., the LED dies 10 a and 10 b) coupled together in series andconnected directly to an AC power supply. This configuration, known as“basic direct AC”, has a low overall efficiency because the individualLEDs produce virtually no light except for when the AC voltage is nearits peak value. That is, the LEDs only operate when there is sufficientvoltage to power all of the LEDs in the device. Other AC-driven LEDdevices include a variable length chain of LED dies, in which one ormore LEDs are switched on as the voltage increases. This configuration,known as “multi-tap direct AC”, provides greater overall powerutilization than basic direct AC systems because at least a portion ofthe LEDs are driven at lower currents.

Both the basic direct and multi-tap LED device arrangements typicallyinclude a large number of LED dies. However, for certain applicationssuch large numbers of LED dies render the use of AC power infeasible.For example, the total emitter area (i.e., the area of a device providedfor the light emitting source) in spot lighting is typically constrainedto such a small area relative to the overall optics that often timesonly a single LED die is used. Achieving the same lumen density with theassembly of LED dies used in conventional AC power approaches wouldtherefore be unfeasible.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a partially schematic cross-sectional diagram of an LED dieconfigured in accordance with the prior art.

FIG. 1B is a partially schematic cross-sectional diagram of an LED dieconfigured in accordance with another embodiment of the prior art.

FIG. 2A is a partially schematic circuit diagram of an SST systemconfigured in accordance with embodiments of the present technology.

FIG. 2B is a partially schematic backside plan view illustrating acontact arrangement for an SST device configured in accordance withembodiments of the present technology.

FIG. 3 is a time versus voltage graph illustrating an AC power curve asit relates to an SST device configured in accordance with embodiments ofthe present technology.

FIG. 4 is a time versus voltage graph illustrating an AC power curve asit relates to an SST device configured in accordance with otherembodiments of the present technology.

FIG. 5 is a schematic view of a system that includes an SST deviceconfigured in accordance with embodiments of the present technology.

DETAILED DESCRIPTION

Specific details of several embodiments of multi junction SST devicesfor use with AC power sources and associated systems and methods aredescribed below. The term “SST” generally refers to solid-statetransducers that include a semiconductor material as the active mediumto convert electrical energy into electromagnetic radiation in thevisible, ultraviolet, infrared, and/or other spectra. For example, SSTsinclude solid-state light emitters (e.g., LEDs, laser diodes, etc.)and/or other sources of emission other than electrical filaments,plasmas, or gases. SSTs can also include solid-state devices thatconvert electromagnetic radiation into electricity. A person skilled inthe relevant art will also understand that the technology may haveadditional embodiments, and that the technology may be practiced withoutseveral of the details of the embodiments described below with referenceto FIGS. 2A-5.

FIG. 2A is a partially schematic circuit diagram of an SST system 200configured in accordance with embodiments of the present technology. TheSST system 200 can include an SST device 202 having a plurality of multijunction SST chains (identified individually as first through nth multijunction SST chains 204 a-204 n, respectively, and referred tocollectively as multi junction SST chains 204 or chains 204) and adriver 206 operably coupled to the individual multi junction SST chains204. The driver 206 can be configured to drive each multi junction SSTchain 204 independently of the others depending on the input voltagereceived from a power source (not shown). For example, the driver 206can be configured to activate or drive the first multi junction SSTchain 204 a when voltage input is at least equal to the drive voltage ofthe first multi junction SST chain 204 a and, as voltage increases(e.g., as it would during a portion of a phase of an AC waveform), thedriver 206 can be configured to activate additional and/or differentchains 204 of the SST device 202. In other embodiments, the driver 206can selectively activate one multi-junction SST chain at a timedepending on the input voltage. In either embodiment, the driver 206 canbe configured to control the SST device 202 such that the absolutevoltage input at a given time is greater than the summation of the drivevoltages of all the activated multi junction SST chains 204.

The multi junction SST chains 204 can be formed together as part of asingle die to provide a small overall emissions area (e.g., as comparedto multi-die devices). In the illustrated embodiment, the SST device 202includes five multi junction SST chains 204 on a single die. In otherembodiments, the SST device 202 can include fewer than five or more thanfive multi-junction SST chains 204 on a single die. In otherembodiments, the multi junction SST chains 204 may be individuallyformed on separate dies.

Each multi junction SST chain 204 can include two or more P-N junctions208 (illustrated as LEDs) coupled together in series (e.g., as shown inFIG. 2A) or in parallel. The individual P-N junctions 208 can include afirst semiconductor material, an active region, and a secondsemiconductor material stacked sequentially on one another and formedusing metal organic chemical vapor deposition (“MOCVD”), molecular beamepitaxy (“MBE”), liquid phase epitaxy (“LPE”), hydride vapor phaseepitaxy (“HVPE”), and/or other suitable epitaxial growth techniquesknown in the art. The first semiconductor material can include a P-typesemiconductor material (e.g., a P-type gallium nitride (“P-GaN”)), andthe second semiconductor material can include an N-type semiconductor(e.g., an N-type gallium nitride (“N-GaN”)). In selected embodiments,the first and second semiconductor materials can individually include atleast one of gallium arsenide (GaAs), aluminum gallium arsenide(AlGaAs), gallium arsenide phosphide (GaAsP), gallium (III) phosphide(GaP), zinc selenide (ZnSe), boron nitride (BN), aluminum galliumnitride (AlGaN), and/or other suitable semiconductor materials. Theactive region can include a single quantum well (“SQW”), MQWs, and/or abulk semiconductor material. The term “bulk semiconductor material”generally refers to a single grain semiconductor material (e.g., InGaN)with a thickness between approximately 10 nanometers and approximately500 nanometers. In certain embodiments, the active region can include anInGaN SQW, GaN/InGaN MQWs, and/or an InGaN bulk material. In otherembodiments, the active region can include aluminum gallium indiumphosphide (AlGaInP), aluminum gallium indium nitride (AlGaInN), and/orother suitable materials or configurations. In various embodiments, theindividual P-N junctions 208 can be configured to emit light in thevisible spectrum (e.g., from about 390 nm to about 750 nm), in theinfrared spectrum (e.g., from about 1050 nm to about 1550 nm), and/or inother suitable spectra. Further aspects of the manner in which the multijunction SST chains 204 can be formed are set forth in U.S. patentapplication Ser. No. 13/210,249, which is incorporated herein byreference in its entirety.

Each P-N junction 208 can have a predefined drive voltage (i.e., thevoltage required to illuminate or otherwise activate the P-N junction208). When the P-N junctions 208 are coupled together in series, thecorresponding multi junction SST chain 204 can have a total drivevoltage that is equal to the sum of the drive voltages of all the P-Njunctions 208 in the multi junction SST chain 204. For example, incertain embodiments the individual multi junction SST chains 204 eachhave four P-N junctions 208, and each P-N junction has a drive voltageof about 3V, resulting in a 12V drive voltage for each multi junctionSST chain 204. In other embodiments, the multi-junction SST chains 204can include different numbers of P-N junctions 208 and/or the individualP-N junctions 208 can have different drive voltages. In furtherembodiments, the individual multi-junction SST chains 204 in the SSTdevice 202 can have different drive voltages. For example, the firstmulti junction SST chain 204 a can have a 12V drive voltage, the secondmulti junction SST chain 204 b can have a 24V drive voltage, and theother multi junction SST chains 204 can have increasingly higher drivevoltages.

The SST device 202 can include multiple terminals or contactscorresponding to the individual multi junction SST chains 204 such thatthe driver 206 can selectively activate the individual multi junctionSST chains 204 independently of each other. FIG. 2B, for example, is abottom view of the SST device 202 configured in accordance with one suchcontact arrangement. Each multi junction SST chain 204 includes aP-contact (identified individually as first through nth P-contactsPa-Pn, respectively, and referred to collectively as P-contacts P) andan N-contact (identified individually as first through nth N-contactsNa-Nn, respectively, and referred to collectively as N-contacts N). Incertain embodiments, the P- and N-contacts can be accessible from thesame side (e.g., the back side) of the SST device 202. As shown in FIG.2B, for example, the P- and N-contacts can be defined by portions of anelectrically conductive substrate 210 (e.g., plated onto the backside ofthe SST device 202), and electrically isolated from each other and theremaining portions of the conductive substrate 210 by isolators 212. Forexample, referring to FIGS. 2A and 2B together, the P-contact Pa can beelectrically coupled to the P-type GaN of the first P-N junction 208 ofthe multi junction SST chain 204 a, and the N-type GaN of the last P-Njunction 208 of the multi junction SST chain 204 a can be electricallycoupled to the N-contact Na, with the intermediate P-N junctions 208serially coupled there between. The remaining multi-junction SST chains204 b-204 n can be electrically coupled to P-contacts Pb-Pn andN-contacts Nb-Nn, respectively. The isolators 212 can be formed by firstcreating two vias associated with each chain 204: one extending throughthe substrate 210 to the P-type GaN of the first P-N junction 208, andthe other that extends through the substrate 210 to the N-type GaN ofthe last P-N junction 208. After the vias have been formed, aninsulative material (e.g., a dielectric material) can be disposed in thevias to isolate the exposed P-type and N-type GaN materials from otherportions of the SST device 202 and define the master contacts of eachmulti junction SST chain 204. In this embodiment, the electricallyconductive substrate 210 can also serve as a thermal pad to transferheat away from the active regions of the multi junction SST chains 204and lower the overall operating temperature of the SST device 202.Further aspects of such plated backside contacts are described in U.S.patent application Ser. No. 13/218,289, which is incorporated byreference herein in its entirety. In other embodiments, the multijunction SST chains 204 can have other suitable contacts that canelectrically connect the individual multi junction SST chains 204 to thedriver 206.

The driver 206 can be electrically coupled to the individual P- andN-contacts with separate outputs (e.g., lines or channels) correspondingto each multi junction SST chain 204. For example, the driver 206 can beelectrically coupled to the P- and N-contacts using backside bondingtechniques, wire bonds, solder bumps, a flip chip arrangement,through-substrate vias, and/or other suitable connection means forelectrically coupling a die to a driver. In operation, current flowsfrom the individual P-contacts to the corresponding N-contacts when thepolarity of the phase of the AC waveform is positive. In certainembodiments, the driver 206 can be configured to reverse the polarity ofone of the phases of the AC voltage (e.g., from negative to positive)such that current always has the same polarity. For example, thenegative polarity phase of the AC waveform can be reversed to bepositive such that current flows from the P-contacts to thecorresponding N-contacts regardless of the phase of the input voltage,and allows the SST device 202 to operate when the phase of the ACvoltage is negative. In other embodiments, the SST device 202 canfurther include separate multi junction SST chains oriented in theopposite direction of those shown in FIG. 2A, such that current can flowin the opposite direction (e.g., as indicated by the arrows of FIG. 2B)when the polarity of the phase of the AC voltage changes. For example,in embodiments operating under AC power, the driver 206 can beconfigured to direct current in a first direction when the phase of theAC voltage is positive, and in a second direction opposite the firstdirection when the phase of the AC voltage is negative.

The driver 206 can be a multi-tap direct-AC driver that selectivelyactivates the individual multi junction SST chains 204 (e.g., via theindividual P- and N-contacts) depending on the power input (e.g., usinga controller integrated with the driver 206). For example, the firstmulti junction SST chain 204 a can be illuminated or otherwise activatedat lower input voltages (e.g., once the voltage supplied is at lestequal to the drive voltage of the first multi junction SST chain 204 a).As the input voltage increases to a predetermined voltage level (e.g.,corresponding to the drive voltage associated with the sum of the firstand second multi junction SST chains 204 a and 204 b), the first andsecond multi junction SST chains 204 a and 204 b can be illuminatedtogether. Additional multi junction SST chains 204 can be powered on andadded to the first and second multi junction chains 204 a and 204 b asthe voltage increases until the maximum voltage is reached and all multijunction SST chains 204 are activated.

In other embodiments, each multi junction SST chain 204 may have adifferent drive voltage, and the driver 206 can selectively activate oneor more individual multi junction SST chains 204 based on the varyinginput voltage. For example, the first through nth multi junction SSTchains 204 a-n can each have different drive voltages (e.g.,incrementally increasing drive voltages than the proceeding multijunction SST chain 204). As the input voltage increases and decreases(e.g., in a sinusoidal manner), the driver 206 can selectively activateand deactivate different multi junction SST chains 204 based on thepredetermined voltage level required to activate each multi junction SSTchain 204. Both configurations of the SST system 200 enable the SSTdevice 202 to directly connect to AC power, and do so while utilizingthe varying levels of available power. For example, unlike conventionalsystems that only activate an SST device when the voltage is sufficientto power all the SSTs in the system, the SST system 200 can operate atleast a portion of the SST device 202 (e.g., one or more multi junctionSST chains 204) at lower voltage levels. Therefore, the SST device 202provides a more efficient power use than conventional AC systems.

In addition, as shown in FIG. 2A, the plurality of multi junction SSTchains 204 can be formed as part of a single die, and therefore take upmuch less area than a plurality of individual SST dies coupled to thedriver 206. The plurality of P-N junctions 208 in each multi junctionSST chain 204 also provide greater overall emissions (e.g., luminousflux) than a single P-N junction can provide. Accordingly, one or moreof the multi junction SST chains 204 can provide greater overallemissions from a smaller area than a conventional single P-N junctionSST dies. This more compact design allows the SST device 202 to be usedin systems with tight space constraints for the emitter (e.g., spotlighting), while still providing a high overall output (e.g., luminousflux).

FIGS. 3 and 4 are time versus voltage plots illustrating sinusoidal ACpower curves as they relate to an SST device (e.g., the SST device 202of FIGS. 2A and 2B) configured in accordance with embodiments of thepresent technology. For illustrative purposes, the embodiments shown inFIGS. 3 and 4 are described with reference to an SST device with threemulti junction SST chains (e.g., configured similar to the multijunction SST chains 204 of FIG. 2A), but in other embodiments SSTdevices can include two or more than three multi-junction SST chains. Inthe embodiment illustrated in FIG. 3, a driver (e.g., the driver 206 ofFIG. 2A) is configured to serially activate multi junction SST chains asthe absolute input voltage increases, and serially deactivate multijunction SST chains as the absolute input voltage decreases. Forexample, the driver can activate a first chain after the input voltagereaches a first predetermined voltage level V₁ (e.g., 12V) correspondingto the voltage necessary to drive the first chain, and the first chaincan remain active as the input voltage increases (e.g., as indicated bythe section of the graph labeled C₁). When the input voltage reaches asecond predetermined voltage level V₂ (e.g., 24V) corresponding to thevoltage necessary to drive the first chain and a second chain together,the first and second chains can be activated (e.g., as indicated by thesection of the graph labeled C₁₊₂). Similarly, a third chain can beactivated once the input voltage is at least equal to a thirdpredetermined voltage level V₃ (e.g., 48V) corresponding to the voltagenecessary to simultaneously drive the first through third chains (e.g.,as indicated by the section of the graph labeled C₁₊₂₊₃). The individualchains can have the same drive voltages, or the drive voltages maydiffer between chains. In embodiments including additional chains, thedriver can be configured to activate an increasing number of chainsuntil all the chains are activated or the maximum input voltage isreached.

As further shown in in FIG. 3, the third chain can be deactivated whenthe input voltage decreases below the third predetermined voltage levelV₃, and the second chain can be deactivated when the input voltagedecreases below the second predetermined voltage level V₂. As the powercurve decreases further and the input voltage becomes negative, thedriver can be configured to reverse the polarity of the input voltageand activate the chains in a similar fashion as when the voltage ispositive (e.g., activating an increasing number of chains as theabsolute voltage increases). In other embodiments, the SST device may beconfigured to operate in the reverse direction when the input voltage isnegative to utilize the negative power input without requiring thedriver to reverse polarity.

In the embodiment illustrated in FIG. 4, each multi junction SST chainhas a different drive voltage (e.g., 6V, 12V, 24V, 40V, 48V, 80V, etc.),and the SST device is configured to activate one chain at a timedepending upon the input voltage. In this embodiment, the first throughthird predetermined voltage levels V₁-V₃ corresponds to the drivevoltage of the first through third chains, respectively. Accordingly,the driver can be configured to activate the first chain alone when theabsolute input voltage is at least equal to the first predeterminedvoltage level (e.g., as indicated by the section of the graph labeledC₁). Once the absolute input voltage increases to the second voltagelevel V₂, the driver can deactivate the first chain and activate thesecond chain (e.g., as indicated by the section of the graph labeledC₂). Similarly, when the absolute input voltage is at least equal to thethird predetermined voltage level V₃, the driver can deactivate thesecond chain and activate the third chain (e.g., as indicated by thesection of the graph labeled C₃). Accordingly, as absolute input voltagedecreases, the chains with higher drive voltages can be deactivated andthe chains with lesser drive voltages can be activated. Both embodimentsshown in FIGS. 3 and 4 can be used directly with AC power sources toutilize the available power as it increases and decreases, and thereforeprovide for a more efficient system than conventional direct-AC system.

In various embodiments, the SST devices described with reference toFIGS. 3 and 4 can also be configured for use with DC power. For example,the SST devices can be connected to a DC power source (e.g., a driverhaving an AC/DC converter) having a specific voltage (e.g., 50V, 80V,100V, etc.), and the driver can selectively activate one or more of thechains based on the voltage of the power source. For example, the drivercan activate the chain or combination of chains with the highest drivevoltage that can be activated by the input voltage supplied by the DCpower source. If a different DC power source with a different voltage isconnected to the SST systems, the driver can selectively activate one ormore chains with complimentary drive voltages. Accordingly, the SSTdevices described with respect to FIGS. 3 and 4 may be compatible foruse with various different DC power sources.

Any one of the SST devices described above with reference to FIGS. 2A-4can be incorporated into any of a myriad of larger and/or more complexsystems, a representative example of which is system 500 shownschematically in FIG. 5. The system 500 can include one or more SSTdevices 510, a driver 520, a processor 530, and/or other subsystems orcomponents 540. The resulting system 500 can perform any of a widevariety of functions, such as backlighting, general illumination, powergenerations, sensors, and/or other suitable functions. Accordingly,representative systems 500 can include, without limitation, hand-helddevices (e.g., mobile phones, tablets, digital readers, and digitalaudio players), lasers, photovoltaic cells, remote controls, computers,and appliances. Components of the system 500 may be housed in a singleunit or distributed over multiple, interconnected units (e.g., through acommunications network). The components of the system 500 can alsoinclude local and/or remote memory storage devices, and any of a widevariety of computer readable media.

From the foregoing, it will be appreciated that specific embodiments ofthe technology have been described herein for purposes of illustration,but that various modifications may be made without deviating from thedisclosure. For example, the SST device 202 illustrated in FIG. 2Aincludes an equal number of P-N junctions 208 in each multi junction SSTchain 204. In other embodiments, however, the individual multi junctionSST chains 204 of an SST device may include different numbers of P-Njunctions. Certain aspects of the new technology described in thecontext of particular embodiments may be combined or eliminated in otherembodiments. Additionally, while advantages associated with certainembodiments of the new technology have been described in the context ofthose embodiments, other embodiments may also exhibit such advantages,and not all embodiments need necessarily exhibit such advantages to fallwithin the scope of the technology. Accordingly, the disclosure andassociated technology can encompass other embodiments not expresslyshown or described herein.

1. A solid-state transducer (SST) system, comprising: an SST devicecomprising a plurality of multi junction SST chains, each multi junctionSST chain having a P-contact, an N-contact, a P-N junction, andconfigured to be illuminated by a drive voltage that activates the P-Njunction; and an AC driver independently coupled to the individual multijunction SST chains, wherein the AC driver is configured to seriallyactivate additional multi junction SST chains as absolute voltage inputincreases, wherein the AC driver is configured to serially deactivatethe multi junction SST chains as absolute voltage input decreases, andwherein the absolute voltage input at a certain time is greater than asum of the drive voltages of all the active multi junction SST chains.2. The SST system of claim 1 wherein the plurality of multi junction SSTchains comprises a first multi junction SST chain to an n^(th) multijunction SST chain, and wherein the drive voltage of each multi junctionSST chain increases from the first multi-junction SST chain to then^(th) multi junction SST chain.
 3. The SST system of claim 1 whereinthe drive voltages of the individual multi junction SST chains aresubstantially equal.
 4. A solid-state transducer (SST) system,comprising: an SST device comprising a plurality of multi junction SSTchains, each multi junction SST chain having a P-contact, an N-contact,a P-N junction, and configured to be illuminated by a drive voltage thatactivates the P-N junction, wherein the drive voltage of at least two ofthe plurality of multi junction SST chains differs from each other; anda driver operably coupled to the individual multi junction SST chains,wherein the driver is configured to activate one multi junction SSTchain at a time depending on voltage input.
 5. The SST system of claim 4wherein the driver is a DC driver.
 6. The SST system of claim 4 wherein:the driver is an AC driver; and the plurality of multi junction SSTchains comprises a first multi junction SST chain to an n^(th) multijunction SST chain, the first multi junction SST chain having the lowestdrive voltage, the n^(th) multi junction SST chain having the highestdrive voltage, and intermediate multi junction SST chains havingincreasingly higher drive voltages from the first to n^(th) multijunction SST chain.
 7. A method of making a solid-state transducer (SST)system, the method comprising: electrically coupling a first multijunction SST chain to a driver; and electrically coupling a second multijunction SST chain to the driver, wherein the driver is configured toactivate the first multi junction SST chain when voltage input is atleast equal to a first predetermined voltage level associated with thefirst multi junction SST chain and activate the second multi junctionSST chain or the first and second multi junction SST chains whenabsolute voltage input increases to a second predetermined voltagelevel.
 8. The method of claim 7 wherein: the first multi junction SSTchain has a first drive voltage; the second multi junction SST chain hasa second drive voltage higher than the first drive voltage; and thedriver is configured to activate the second multi junction SST chainalone when absolute voltage input is at least equal to the second drivevoltage.
 9. The method of claim 7 wherein: the first multi junction SSTchain has a first drive voltage; the second multi junction SST chain hasa second drive voltage; and the driver is configured to drive the firstand second multi junction SST chains when absolute voltage input is atleast equal to a summation of the first and second drive voltages. 10.The method of claim 7, further comprising: electrically coupling a thirdmulti junction SST chain to the driver, wherein individual SST junctionsof the third multi junction SST chain are oriented in an oppositedirection to the individual SST junctions of the first and second multijunction SST chains; and wherein the driver is configured to drive thefirst and second multi junction SST chains in a first direction when aphase of AC voltage input is positive and drive the third multi junctionSST chain in a second direction opposite the first direction when thephase of the AC voltage input is negative.
 11. The method of claim 7wherein: the first and second multi junction SST chains are two of aplurality of multi junction SST chains that define an SST device; andthe driver is an AC driver, wherein the AC driver is configured toserially activate the multi junction SST chains as absolute voltageinput increases, and wherein the AC driver is configured to seriallydeactivate the multi junction SST chains as absolute voltage inputdecreases.
 12. A method of operating a solid-state transducer (SST)system, the method comprising: receiving a voltage at driver; activatinga first multi junction SST chain of a plurality of multi junction SSTchains when absolute voltage input is at least equal to a firstpredetermined voltage level, wherein the first predetermined voltagelevel is at least equal to a drive voltage of the first multi junctionSST chain; and activating a second multi junction SST chain of P-Njunctions of the plurality of multi-junction SST chains of P-N junctionswhen absolute voltage input is at a second predetermined voltage level,wherein the second predetermined voltage level is at least equal to adrive voltage of the second multi junction SST chain, and wherein thesecond predetermined voltage level is higher than the firstpredetermined voltage level.
 13. The method of claim 12 wherein thedrive voltage of the second multi-junction SST chain is higher than thedrive voltage of the first multi junction SST chain, and wherein themethod further comprises deactivating the first multi junction SST chainwhen the second multi junction SST chain is activated.
 14. The method ofclaim 12 wherein activating the second multi junction SST chaincomprises activating the second multi junction SST chain at the sametime as the first multi junction SST chain, and wherein the methodfurther comprises: activating an increasing number of the multi junctionSST chains in the plurality of multi junction SST chains as absolutevoltage input increases; and deactivating an increasing number of themulti junction SST chains as absolute voltage input decreases such thatthe absolute voltage input at a certain time is greater than a sum ofthe drive voltages of all the active multi junction SST chains.
 15. Themethod of claim 12, further comprising: driving a first portion of themulti junction SST chains in a first direction during a positivepolarity phase of an AC voltage waveform; and driving a second portionof the multi junction SST chains in a second direction opposite thefirst direction during a negative polarity phase of the AC voltagewaveform.